The protein phosphorylation/dephosphorylation cycle is one of the major regulatory mechanisms employed by eukaryotic cells to control cellular activities. See U.S. Pat. No. 5,853,997. It is estimated that more than 10% of the active proteins in a typical mammalian cell are phosphorylated. During protein phosphorylation/dephosphorylation, phosphate groups are transferred from adenosine triphosphate molecules to a protein by protein kinases and are removed from the protein by protein phosphatases.
Protein phosphatases function in cellular signaling events that regulate cell growth and differentiation, cell-to-cell contacts, the cell cycle, and oncogenesis. Three protein phosphatase families have been identified as evolutionarily distinct. These include the serine/threonine phosphatases, the protein tyrosine phosphatases, and the acid/alkaline phosphatases (Carbonneau & Tonks, Ann. Rev. Cell Biol. 8, 463–93, 1992).
The serine/threonine phosphatases are either cytosolic or associated with a receptor. On the basis of their sensitivity to two thermostable proteins, inhibitors 1 and 2, and their divalent cation requirements, the serine/threonine phosphatases can be separated into four distinct groups: PP-I, PP-IIA, PP-IIB, and PP-IIC. PP-I dephosphorylates many of the proteins phosphorylated by cyclic AMP-dependent protein kinase and is therefore an important regulator of many cyclic AMP mediated, hormone responses in cells. PP-IIA has broad specificity for control of cell cycle, growth and proliferation, and DNA replication and is the main phosphatase responsible for reversing the phosphorylations of serine/threonine kinases. PP-IIB, or calcineurin (Cn), is a Ca+2-activated phosphatase; it is involved in the regulation of such diverse cellular functions as ion channel regulation, neuronal transmission, gene transcription, muscle glycogen metabolism, and lymphocyte activation. PP-IIC is a Mg+2-dependent phosphatase which participates in a wide variety of functions, including regulating cyclic AMP-activated protein-kinase activity, Ca+2-dependent signal transduction, tRNA splicing, and signal transmission related to heat shock responses. PP-IIC is a monomeric protein with a molecular mass of about 40–45 kD. One α and several β isoforms of PP-IIC have been identified (Wenk et al., FEBS Lett. 297, 135–38, 1992; Terasawa et al., Arch. Biochem. Biophys. 307, 342–49, 1993; and Kato et al., Arch. Biochem. Biophys. 318,387–93, 1995).
Because of the importance of protein phosphatases in a variety of biological functions, there is a need in the art to identify additional protein phosphatases which can be regulated to provide therapeutic effects.